1. Field of the Invention
The present invention relates to temperature control systems and more particularly to a system having a means for shifting the temperature control point in accordance with outdoor ambient temperature and the K-Factor of the room.
2. Description of the Prior Art
It is well known in the fields of heating, ventilating and air-conditioning that the number of BTU's required to maintain a room at a particular temperature is proportional to the difference between indoor and outdoor temperatures and a factor known as the K-Factor. The BTU's required to maintain a particular temperature may be expressed as: BTU=K (T.sub.indoor -T.sub.outdoor) where the K is the overall heat transfer coefficient expressed in units of (BTU/Hr. Ft.sup.2 .degree. F). This factor usually has a value in the range of 0.1 to 1.2 for a room and is calculated by the architect for each room of a building. Thus, the BTU's required to maintain a particular room temperature is directly related to the heat transferred through the walls, windows, etc. of the room and will vary considerably from room to room. A more generalized equation would include the effects of heat transfer through interior surfaces, internal heat gain due to people, lights and equipment and also temperature changes caused by air leakages through the room surfaces and fresh air introduced into the room by ventilating equipment.
Prior art devices have attempted to introduce a control signal corresponding to the outdoor air temperature into the temperature control system to compensate for heat transfer through the walls of the building. However, the amount of adjustment required to compensate for the heat transfer was never accurately known and was usually introduced into the system based on average requirements on a compromise basis.
In prior art devices such as home heating thermostats, there is no compensation for heat loss caused by reduced outdoor temperature and the occupant usually shifts the thermostat set point upwardly to avoid becoming chilled. This type of manual compensation is not practical for use in public buildings where the set point must be fixed.
An an example, in prior art devices the temperature of the room usually had to drop by 21/2.degree. to the temperature control point before full heating was called for. This was not a particular problem when a 72.degree. heat set point was used since the full heat would be provided when the temperature dropped to a control point temperature of 691/2.degree. which is within the comfort range of most occupants. However, with the advent of the energy crisis and the emphasis being placed on energy conservation, it has become mandatory that the heat set point be reduced to 68.degree. in many buildings. When the heat set point is set at 68.degree., the room temperature must drop to a control point temperature of 651/2.degree. before full heat is called for. A temperature of 651/2.degree. is below the comfort range for most persons and will be unacceptable. Thus, it is apparent that the prior art devices without compensation for heat transfer through the walls of the building are unsatisfactory when using a 68.degree. heat set point.
In like manner, the cooling set point has been removed upwardly in most public buildings and the heat entering the room through the walls can raise the inside temperature to a very uncomfortable level unless compensation is provided that calls for cooling to prevent an undue increase in room temperature due to heat transfer through the walls.
It is apparent that when a compromise solution is used that provides compensation based only on outdoor temperature, excess heating or cooling will be called for in rooms where only a small amount of heat is transferred through the walls and in like manner insufficient heating or cooling will be supplied to rooms where there is excessive transfer of heat through the walls. The net result being that certain rooms will be uncomfortable, while other rooms will consume energy unnecessarily.